*{
    padding:0px;
    margin:0px;
}
.out{		/*最外面的div*/
    background-color:gray;
    height:90%;
    width:85%;
    position:relative;
}
.inside{   /*在div类名为out里的div*/
    height:60%;
    width:70%;
    margin:auto;
    background-color:yellow;
    top:0px;
    bottom:0px;
    left:0px;
    right:0px;
    position:absolute;
}
body{
    height:300px;
    width:400px;
}